Procedure and device for the evaluation of the quality of a cable

ABSTRACT

A procedure and a system for the evaluation of the quality and/or efficiency of a cable or a cable segment by a current measurement is disclosed. This is achieved by supplying between the core and the screen of a cable a voltage with alternating polarity and rectangular shape. The periodic duration&#39; of this voltage is selected in a way so as to permit the current measurement of the charge current shortly before a polarity reversal, providing a current value equal to the leakage current.

RELATED APPLICATIONS

This application claims priority to German application DE 100 47 548.5,filed on Sep. 22, 2000.

BACKGROUND

1. Field of the Invention

This invention concerns a procedure for the evaluation of the qualityand/or the efficiency of a cable in a cable installation by a currentmeasurement of a leakage current as a parasitic cross current.

2. Discussion of Related Art

It is known, that the parasitic cross current flows as a leakage currentbetween a current carrying conductor at the core of a cable and thecable screen or concentric, which is generally an earth-groundpotential. This current reduces the efficiency and/or the Quality of thetransmission. By the measurement of this leakage current, the quality ofthe cable can be determined, additionally a corresponding test criteriacan be established. The higher the leakage current, the lower is thequality of the cable.

For the evaluation of the leakage current, it was recommended to connecta DC voltage between the core and the screen of the cable and to measurethe resulting flow of the current. For this purpose the cable isdisconnected from the net and tested with a DC voltage equivalent to theoperating voltage level. The leakage current is measured via a connectedcurrent meter. The disadvantage of this procedure is the generation ofpolarisation effects in the dielectric of the cable insulation resultingin a possible pre-damaging of the cable.

Further known methods are the so called VLF (very low frequency) Testmethods. For this method, an alternating voltage with a very lowfrequency, for example 0.1 Hz is applied. The rising/falling slope ofthis frequency is in the frequency spectrum of 50 Hz. Significantdisadvantages of this technology are interferences of compensationlosses and leakage current which prevent a stable and continuousmeasurement of the leakage current.

Further known technologies as resonance testing have the samedisadvantage, that the continuous measurement of the leakage current isnot possible. This is a result of continuous re-charging procedures thatprevent a stable situation in the cable.

SUMMARY

The task of the invention is therefore to provide a procedure and adevice of the previously described type, which provides a significantlymore reliable evaluation of the quality and/or efficiency of a cable orcable section than previously proposed systems.

A procedure for the evaluation of the quality and/or the efficiency of acable or a cable segment in accordance with the present inventionincludes a current measurement of a leakage current indicating theparasitic cross current. This measurement is achieved by supplyingbetween two conductors for example the core and a screen or two cores ofa cable, a voltage (V_(VLF)) with alternating polarity and rectangularshape. The periodic duration of the voltage (V_(VLF)) is selected in away which permits the current measurement of the leakage current at atime shortly before a polarity reversal where the voltage change or riseis zero (ΔV=0). The leakage current close to the polarity reversal issubstantially the same as the parasitic cross current. In someembodiments, the current measurement of the leakage current is performedat a selected time period.

A device on which this procedure can be performed includes a VLFGenerator for the generation of a VLF Voltage with changing polarity insubstantially a rectangular shape. The VLF voltage is coupled to a cableto be tested and an Analyzing and Evaluation unit for the evaluation andvisualization of a leakage current in the tested cable shortly before apolarity change of the VLF voltage.

Further details, characteristics and advantages can be deduced from thefollowing description with respect to the following figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a principle diagram of a measuring arrangement according tothe present invention.

FIGS. 2a and 2 b show current and voltage diagrams of the measuringarrangement shown in FIG. 1.

FIG. 3 shows a measurement of parasitic current between centralconductors of adjacent cables.

FIG. 4 shows an embodiment of the analysis and evaluation unit of FIGS.1 and 3.

FIGS. 5A and 5B show embodiments of an algorithm executed on theanalysis and evaluation unit of FIG. 4.

In the figures, elements designated with the same identification havethe same function.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a measurement system according to thepresent invention. The measurement system of FIG. 1 includes a VLFGenerator 10, a Voltage measuring device (Voltmeter) 11, a currentmeasuring device (Amperemeter) 12, and an Analysis and Evaluation unit13. The VLF Generator feeds a cable 20, where one pole is connected to acore 21 of cable 20 and the other pole is connected to a screen 22 ofcable 20. Voltmeter 11 measures the voltage at the VLF generator, andresultingly between core 21 and screen 22 of cable 20. Amperemeter 12measures the current flowing into the core 21. The measured voltage andcurrent values are supplied to the Analysis and Evaluation unit 13.

Although FIG. 1 shows generator 10 coupled between core 21 and sheath 22of cable 20, a parasitic current between adjacent cores 21 and 24 incable 20 can also be measured. FIG. 3 shows a measurement systemaccording to the present invention for measuring the parasitic currentbetween adjacent cores 21 and 24 in cable 20 in the same fashion as theparasitic current between the inner conductor and outer sheath or soilground is measured shown in FIG. 1.

The shape of the voltage generated by VLF Generator 10 is shown in FIG.2b. FIG. 2b shows a square wave voltage, or a shaped cosine square wave.The shape of the according resulting current from this voltage, flowinginto core 21 of cable 20, is shown in FIG. 2a. As shown in FIG. 2a, thecurrent I rises with a polarity change of the VLF voltage from positiveto negative potential to a maximum cable charge current i₁ and decreasesfollowing down to a leakage current I₀ before the next polarity change.The next polarity change, which will be in the reverse direction,results in the corresponding response from the current into negativepolarity.

For the VLF Test, the leakage current is equal to the compensationcurrent, which is supplied in the cable 20 by the VLF Generator 10,shortly before the next polarity change (recharging process).

Analysis and Evaluation unit 13 receives and stores the actual measuredvalue I₀ at a moment when the VLF voltage is stable (ΔV=0). Thiscondition is shortly before the next polarity change and represents acondition where the measured value of the current is equal to theleakage current of cable 20. In FIG. 2a this moment is indicated withthe notation t_(A). The leakage current is measured and indicatedselectively at a specific point and displayed over a long duration. Bythe constant change of the polarity of the VLF Voltage connected to thecable, a damage of the dielectric of the cable insulation is unlikely.

A block diagram of an embodiment of analysis and evaluation unit 13 isshown in FIG. 4. Analysis and evaluation unit 13 of FIG. 4 includesmicrocontroller 404 coupled to analog-to-digital converters 401 and 402and a clock 403. In some embodiments, analysis and evaluation unit 13can be an electronic circuit not including a microcontroller ormicroprocessor.

Analog-to-digital converter 401 receives and digitizes a voltage signalfrom voltmeter 11. Analog-to-digital converter 402 receives anddigitizes a current signal from current meter 12. Analysis and displayof the leakage current in cable 21 is performed in microcontroller 404.

FIGS. 5A and 5B show embodiments of algorithm 500 which can be performedon microcontroller 404 according to the present invention. Algorithm 500shown in FIG. 5A starts in start block 501. In block 502, the voltagefrom voltmeter 11 is checked to be sure that it is substantially areference voltage (e.g., within about 10%). Algorithm 500 waits untilthe condition of block 502 is met. In block 503, algorithm 500 waits fora trigger impulse. The trigger impulse can be set at a time where thechange in voltage will be approximately zero. In some embodiments, thetrigger impulse is timed with the clock signal from clock 403. Algorithm500 then waits until the trigger impulse is received.

In block 504, the current from amperemeter 12 is measured. In block 505,the current measured in block 504 is adjusted for normal charging ofcable 20 or for current supplied by generator 10 that is not leakagecurrent. In step 506, the results of the evaluation are displayed.

In algorithm 500 of FIG. 5B, condition block 503 of FIG. 5A is replacedwith condition block 510 of FIG. 5B. Condition block 510 checks to seeif the change in voltage is substantially zero, indicating that cable 20is fully charged. In block 510, the change in voltage is measured andalgorithm 500 waits until the change in voltage is approximately zero.

The above described embodiments of the invention are exemplary only. Oneskilled in the art may deduce various modifications to the embodimentsdescribed here which are intended to be within the scope of thisinvention. As such, the invention is limited only by the followingclaims.

We claim:
 1. A method for the evaluation of the quality and/orefficiency of a cable or a cable segment, comprising: supplying avoltage having an alternating polarity and a rectangular shape in timebetween a first conductor and a second conductor; measuring a parasiticcross current between the first conductor and the second conductor at atime just prior to a polarity change in the voltage; determining thequality and/or efficiency of the cable from the parasitic cross current.2. The method of claim 1, wherein the time just prior to the polaritychange can be characterized as the time when the voltage is stable. 3.The method of claim 1, wherein measuring the parasitic cross-currentincludes measuring the parasitic current at a selected time.
 4. Themethod of claim 1, wherein measuring the parasitic cross-currentincludes measuring the parasitic current at a time when a change in thevoltage is low.
 5. The method of claim 1, wherein the quality of thecable decreases with increasing values of the parasitic cross current.6. The method of claim 1, wherein a period of the alternating polarityis sufficiently long to permit the voltage to stabilize and measurementof the parasitic cross current.
 7. The method of claim 1, wherein thefirst conductor is a core and the second conductor is a sheath aroundthe core.
 8. The method of claim 1, wherein the cable includes at leasttwo core conductors and the first conductor is a first core and thesecond conductor is a second core.
 9. A measuring system, comprising: avoltage generator for generating a voltage with changing polarity insubstantially a rectangular shape, the voltage generator being capableof being coupled between a first conductor and a second conductor of atest cable; an ampere meter coupled between the voltage generator andthe first conductor; an analyzing and evaluation unit coupled to receivea current signal from the ampere meter, the analyzing and evaluationunit measuring the current signal just prior to a polarity change of thevoltage and indicating a quality or efficiency value for the test cable.10. The system of claim 9, further including a voltmeter coupled betweenthe first conductor and the second conductor, the voltmeter providing avoltage measurement to the analyzing and evaluation unit.
 11. The systemof claim 9, wherein the analyzing and evaluation unit determines thecurrent signal corresponding to the parasitic cross current when thevoltage measurement from the voltmeter indicates a stable voltage. 12.The system of claim 11, wherein the stable voltage is correlates with aselected time of measurement.
 13. The system of claim 11, wherein thestable voltage is determined by measuring the change in voltage.
 14. Thesystem of claim 9, wherein the first conductor is a core and the secondconductor is a sheath of the test cable.
 15. The system of claim 9,wherein the first conductor is a core and the second conductor is asecond core of the test cable.
 16. A measuring system, comprising: meansfor applying a substantially rectangular voltage between a firstconductor and a second conductor of a cable; means for measuring aparasitic current between the first conductor and the second conductorjust prior to a change in polarity of the substantially rectangularvoltage; and means for determining cable quality from the parasiticcurrent.
 17. The system of claim 16, wherein the first conductor is acore of the cable and the second conductor is a sheath of the cable. 18.The system of claim 16, wherein the first conductor is a core of thecable and the second conductor is a second core of the cable.
 19. Thesystem of claim 16, further including means for monitoring the voltagebetween the first conductor and the second conductor.
 20. The system ofclaim 16, further including means for determining a stable voltagebetween the first conductor and the second conductor.